Median through C is x=4 So the x coordianate of C is 4 . let C≡(4,y), then the midpoint of A(1,2) and C(4,y) is D which lies on the median through B. 
∴D≡(21+4,22+y) Now 21+4+2+y=5⇒y=3. So, C≡(4,3). The centroid of the triangle is the intersection of the mesians. Here the medians x=4 and x+4 and x+y=5 intersect at G(4,1). The area of triangle △ABC=3×△AGC =3×21[1(1−3)+4(3−2)+4(2−1)]=9.